Modern vehicles remain absolutely dependent on properly functioning components to keep pollution levels low. Minor malfunctions in the air and fuel or spark management systems can increase emissions significantly. Major malfunctions can cause emissions to skyrocket. A relatively small number of vehicles with serious malfunctions frequently cause the majority of the vehicle-related pollution problem. Unfortunately, it is rarely obvious which vehicles fall into this category, as the emissions themselves may not be noticeable and emission control malfunctions do not necessarily affect vehicle driveability. Effective IM programs, however, can identify these problem cars and assure their repair.

For countries with only minimal if any controls on vehicles, a simple IM program can be a good pollution control starting point as even vehicles with no pollution controls can benefit from improved maintenance. A simple idle check on CO and HC missions from gasoline vehicles or visible smoke check on diesel vehicles can be used to identify the highest polluters and those vehicles which would most benefit from remedial maintenance. Several years ago, Hong Kong, whose air quality problem was primarily excess particulate, trained a small group of smoke inspectors who then patrolled the streets, identifying vehicles with excess smoke and requiring them to be repaired or pay a fine. Such a program requires minimal capital investment and resources.

As vehicle technology advances, more sophisticated test procedures may be necessary including loaded mode tests that use a dynamometer to simulate the work that an engine must perform in actual driving.

Substantial advances are occurring in IM programs. For the most advanced vehicles, those equipped with electronic controls of air-fuel and spark management systems and equipped with catalytic converters to reduce CO, HC and NOx, a transient test which includes accelerations and decelerations typical of actual driving can provide additional emissions reduction benefits.

As a general matter, maximum IM effectiveness occurs with centralized IM systems. These programs also cost much less overall and are more convenient to the public.